Answer: Based on your description I am not certain what is happening to the one that is wilting. Mandevillas need ample soil moisture (evenly moist, not sopping wet or saturated) when they are actively growing; they also need rich soil. It is possible the wilting one is not rooting out into the surrounding soil, or perhaps it is receiving a different amount of water. Overwatering can cause damage to the tuber, and overly dry soil will also have a damaging effect. Or, perhaps there is some damage to the main vine stem; if that happened you can cut it back and it should regrow fairly fast due to the large root system. Since you have one that is fine, you are probably taking good care of them. If they are under warranty you might want to consider that option. But, in case there is a disease problem do not plant the replacement in the exact same spot. I should also mention that if you plan to bring them inside in the fall it is usually easier to do so when they are kept in containers year round. Enjoy your mandevillas!
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